This invention relates generally to method and apparatus for optically recording and reproducing information in and from an optical disc, and more particularly, the present invention relates to method and apparatus with which it is possible to rewrite or renew prerecorded information or data.
In known optical disc recording and reproducing systems, a light beam, such as laser light, having a diameter of 1 micrometer or less is incident on a rotating disc having a photosensitive recording material layer thereon. As a result, information is recorded in the form of pits, i.e. concavo-convex portions, holes or light and shade portions. Namely, the energy of the laser light beam is converted into heat so that the optical characteristics, such as the reflection coefficent or the transmittance, are changed to record the information at a high density.
In order to keep accurately the positional relationship between an optical head and the optical disc irrespective of the variations in the optical head feeding mechanism or in the disc drive mechanism, one or more guide tracks which are optically detectable are provided so that tracking control can be effected. As one example of such a tracking control, coaxial or spiral guide tracks are made on the entire area of the information signal recording region on the optical disc where the guide tracks are made of grooves each having a width of approximately 0.5 micrometers and a depth which is approximately one eighth the wavelength of the light used for recording and reproducing information. As another example of tracking control, a servo track carrying a predetermined servo signal is made so that recording and reproducing will be performed along the servo track.
When using such an optical disc having one or more guide tracks, track addresses are assigned respectively to the guide tracks so that recording (writing) and reproducing (reading) of digital information or data can be performed at any desired places on the optical disc. The guide tracks are divided into a plurality of regions which are called sectors so that a unit of information is recorded or reproduced by designating a region or area defined by the track address and the sector address.
The track and sector addresses are recorded in a directory region also made on the optical disc so that the positional relationship between the optical head and a given portion of the disc can be controlled to write or read information on or from the given portion. When it is intended to make some changes in the prerecorded information, however, the prerecorded information cannot be simply erased as in a magnetic recording medium because optically changed portions cannot be restored to the original state. For this reason, when it is desired to rewrite or edit prewritten information on an optical disc, the information has to be read out first from where it has been recorded to be editted, and then the editted or corrected information has to be recorded on a portion where no information has been recorded yet. In other words, the same portion or area on the optical disc cannot be used for repetitive recording. When rewriting or editting, which will be referred to as renewing hereafter, it is necessary to employ an external nonvolatile storage, such as a magnetic disc or the like, to store directories of various pieces of information or data prewritten in an optical disc. Each directory includes a track address and a sector address of a portion or region on an optical disc so that a particular region can be found to reproduce or read data therefrom.
Each of the directories has an identification name or code to constitute an index so that a given unit of information or data can be read out by designating a predetermined identification. Identifications with which directories of information pieces are designated should not be changed even if the contents of information are renewed so that the renewed information can be read out with its original identification. Therefore, in conventional optical recording systems, when one or more pieces of information prerecorded in an optical disc are to be renewed by writing the editted information on an unused portion on the disc, the corresponding directory or directories have to be changed and stored in the external storage again. Namely, in the conventional technique of renewing data on an optical disc, a nonvolatile external storage is needed for each disc to keep the directories of all pieces of information recorded in an optical disc. Accordingly, optical discs have to be used with associated nonvolatile external storages in pairs. For this reason, it has been difficult to handle optical discs. Furthermore, since it has been impossible to erase prewritten information from an optical disc, optical discs have to be kept with care so that secret information once written is not read out by others.